This invention relates generally to a system of data transmission between a network server and a remote user client device and, more particularly, to a system using data streaming to download or transfer large files (principally containing multimedia information) between the network server and the client device.
Currently there are two prevalent methods of downloading multimedia (e.g. audio/video) data files on the Internet. The first method involves using Transmission Control Protocol/Internet Protocol (TCP/IP) to download or deliver an entire digital audio and/or digital video file from a source network server to a remote client device. The client device may be a personal computer equipped with an Internet browser and multimedia playback tools. Once the client device has received the entire file, the user can begin to view or play the audio/video data file locally. However, this method suffers from several limitations, the most important being the need for a very large buffer at the client device.
The second method uses a data streaming technique in which a large multimedia file (which might be three minutes long and may be tens of Mbytes large) on a network server is downloaded to a client device through a constantly flowing stream of data. The client device""s play-out apparatus continuously decodes the audio/video streams and then plays them in real time, on demand. With data streaming, a small portion or xe2x80x9csegmentxe2x80x9d of the data is sent to a client device and stored in a play-out buffer. This segment of data is then rendered (e.g. played, viewed and the like) almost instantaneously. While the client device renders the current segment of downloaded data, the steaming application simultaneously downloads new data to the client device""s buffer. While the streaming technique thus avoids the need for a large buffer, because the data is downloaded on a segment by segment basis, there are still problems experienced by users as a result of network congestion.
Network congestion can be the result of a number of factors, some of which relate to the architecture of the telecommunications network and the arrangement of the network elements, such as the use of undedicated resources, e.g., shared routers, which must compete in a sometimes inefficient manner to effect file transfers and download multimedia streams to users. Congestion also occurs due to traffic on the Internet that has virtually exploded as a result of the overwhelming popularity of the World Wide Web (WWW). Further, network servers connected to the Internet are getting overloaded with other types of queries and information requests, including e-mails, internet telephony, etc.
The current data streaming approach can not adequately account for congestion in the network because, when congestion occurs, the network server cannot continue to effectively and rapidly download the multimedia files and thereby replenish the client device""s play-out buffer before the buffer becomes empty. This is because only a single logical connection is formed between the network server and the client device for data streaming. When network congestion occurs in the connection, the rate at which data is received in the play-out buffer is slower than the rate at which it is extracted from the play-out buffer by the client device. Because the play-out buffer is only intended, with data streaming, to store segments of the multimedia file, the play-out buffer is usually not large enough to absorb the network congestion. In other words, the client device""s play-out buffer will be depleted before the network congestion terminates which can lead to a freezing effect in the streaming applications. When such a freezing effect occurs, the user can experience a momentary and longer-lasting interruption to the multimedia playback experience, hence, the overall playback quality suffers.
From the foregoing, it is seen that although prior art arrangements for data streaming perform satisfactorily in some applications and under some circumstances, inadequacies such as congestion in the communication networks often adversely affect user perceptions of multimedia application performance. Consequently, there is a need to alleviate the problem caused by network congestion when multimedia files are downloaded from a network server to a network user using data streaming.
Multimedia playback is enhanced and the effects of network congestion are significantly reduced over the current art in data streaming arrangements by interposing at least one caching server within a communication network, in the communication path between a network server and a network user""s client device. The caching server(s) in the communication path forms a level or series of levels of caching servers, with level one starting from the caching server nearest the network server and proceeding downstream to level N nearest the client device. As a result, a series of individual logical connections are formed in the communication path, such as (1) the connection between the network server and the level one caching server and (2) the connection between the level N caching server and the client device. In addition, each caching server is arranged to determine whether network congestion exists in the downstream connection of that particular caching server and, if so, to absorb it. To absorb network congestion, each caching server is equipped with an expandable buffer and an arrangement to vary the rate at which data is sent downstream from a particular caching server that experiences congestion in its downstream connection to the next caching server or to the client device.
In operation, a two-phase caching technique is utilized (1) an initialization phase to reduce response time and provide a start-up sequence of data segments closer to the client device and (2) a steady-state phase to continuously stream data segments to the client device. In the initialization phase, a client device requests data located in the network server. The network server can receive the request in a variety of ways, including directly from the client device or from the caching servers in the communication network. In response, the network server segments the requested data and transmits a start-up sequence of data segments to the appropriate caching servers and/or client device, for example, each of the appropriate caching servers and the client device may receive and store one initial data segment. During the initialization phase the present invention allows data to be transmitted and accumulated in servers nearer to the client device than would occur in conventional arrangements.
In the steady-state phase, the start-up data segments that now have been stored are continuously streamed, at a first data rate, through the one or more caching servers and then to the client device to allow a constant multimedia playback performance at the client device. As the client device depletes the initial data segment in its buffer, the client device is replenished with a new data segment by the nearest caching server, and the nearest caching sever is replenished with a new data segment from the next upstream caching server, and so on.
If network congestion should occur during the steady-state phase, each caching server is equipped to absorb the network congestion in its downstream connection. In particular, each caching server periodically determines whether network congestion exists in its downstream connection, and if network congestion exists, the caching server: (1) decreases the data transfer rate from the first data rate to a slower second data rate so that data is not lost due to the congestion and (2) increases the size of its expandable buffer to accommodate additional data segments received from its upstream caching server. When the network congestion subsides or terminates, the caching server (1) increases the data transfer rate, for example, to a third data rate faster than the first data rate to quickly replenish the downstream caching server with new data segments, and (2) decreases the size of its expandable buffer as the additional data segments are depleted. In this manner, the system and method of the present invention improves the performance of data streaming by reducing the freezing effect in the client device, which may result from network congestion in the current single connection streaming approach.